The invention relates to polymers for use in optical devices such as photoluminescent and electroluminescent devices.
Polymer LEDs were first described by Burroughes et al (PCT GB90/00584). Devices based on copolymers (Holmes et al, PCT GB91/01420; PCT GB91/01421) multilayers (PCT GB93/01573; PCT GB93/01574) and with high electron affinity polymers have also been reported (PCT GB94/01118).
Conjugated poly(3-alkylthienylene)s have been prepared, and reviewed by J. Roncali (Chem Rev, 1992, 92, 711) and applications in electroluminescent devices were reported by Y. Ohmori et al. (Jpn, J. Appl. Phys. Part 2, 1991 20(11B), L1938-1940. Regioregular poly(3-alkylthienylene) s have been described by R. D. McCullough, R. D. Lowe, M. Jayaraman, and D. L. Anderson, (J. Org. Chem., 1993, 58, 904). Solvent dependent chiroptical behaviour has been reported for regioregular poly (3-alkylthienylene) s M. M. Bouman, E. E. Havinga, R. A. J. Janssen and E. W. Meijer, Mol. Cryst. Liq. Crist., 1994, 256, 439). Regiorandom hydroxy-functionalised polythiophene copolymers have been reported (C. Della Casa, E. Salatelli, F. Andreani and P. Costa Bizzarri, Makromol. Chem. Makromol. Symp.,1992, 59, 233), and the potential for cross linking was noted (J. Lowe and S. Holdcroft, Polym. Prepr., 1994, 35, 297-298).
More advanced polymeric LEDs can involve the use of both emissive and charge transport materials in order to improve the efficiency of the device [P. L. Burn, A. B. Holmes, A. Kraft, A. R. Brown, D. D. C. Bradley, R. H. Friend, Mat. Res. Soc. Symp. Proc., 1992, 247, 647; A. R. Brown, D. D. C. Bradley, J. H. Burroughes, R. H. Friend, N. C. Greenham, P. L. Burn, A. B. Holmes and A. Kraft, Appl. Phys. Lett., 1992, 61, 2793; T. Nakano, S. Doi, T. Noguchi, T. Ohnishi Y. Iyechika, Sumitomo Chemical Company Limited, U.S. Pat. No. 5,317,169, May 31, 1994]. Emissive polymers are the main active layer in polymer LEDs. Singlet excitons are formed under double charge injection which then decay radiatively to produce light emission. On the other hand, charge transport polymers have also been found to play an important role in enhancing the internal quantum efficiency of devices (photons emitted per electron injected), decreasing working voltages and in increasing the life-time of the device. This was fist shown by use of the known charge transporting molecule (PBD) [2-(4-biphenyl)-5-(4-tert-butyl-phenyl)-1,3,4-oxadiazole] as a blend in poly(methyl methacrylate) as mentioned above [Burn et al.; Nakano et al.]. Recently, high efficiency (4%) blue electroluminescence has been achieved by means of charge-transporting layers using polyvinylcarbozole (PVK) as a hole-transporting material and PBD blended with poly(methyl methacrylate) (PMMA) as an electron transporting material in the multi-layer device [ITO/PVK/PQ (polyquinoline)/PBD+PMMA/Ca] [I. D. Parker, Q. Pei, M. Marrocco, Appl. Phys. Lett., 1994, 65(10), 1272]. The role of the charge transport layer in LEDs include: (i) assisting effective carrier injection from the electrode to the emitting layer (ii) confining the carriers within the emitting layer and thus increasing the probability of recombination processes through radiative decay, leading to light emission (iii) preventing the quenching of excitons at the boundary between an emitting material and the electrode.
Most common conjugated polymers are more easily p-doped and thus exhibit hole-transport properties. On the other hand, electron transport and electron injection in polymer LEDs have proved to be more difficult and are thus required in order to improve device efficiency and performance.
An aromatic oxadiazole compound such as PBD is well known to be a useful electron transport material [K. Naito, Jpn. Kokai Tokkyo Koho, JP 05,202,011,1993; S. Lunak, M. Nepras, A. Kurfurst and J. Kuthan, Chem. Phys., 1993, 170, 67]. Multi-layered LED devices with improved efficiency have been reported using evaporated PBD or a spin-coated PBD/PMMA blend as an electron transport layer.
In each case, however, problems that will lead to device break-down (such as the aggregation and re-crystallisation of PBD) may be expected to occur under the influence of an electrical field or temperature increase when the device is working [C. Adachi, et al, Jpn. J. Appl. Phys., 1988, 27, L269; C. Adachi, S. Tokito, T. Tsutsui, S. Saito, Jpn. J. Appl. Phys.1988, 27, L713; Y. Hamada, C. Adachi, T. Tsutsui, S. Saito, Jpn. J. Appl. Phys. 1992, 31, 1812; K. Naito, A. Miura, J. Phys. Chem., 1993, 97, 6240].
Conjugated polymers that contain aromatic and/or heteroaromatic rings have enjoyed considerable interest because of their potential electrical conductively after being doped and electroluminescent properties. However, there is a severe processibility problem for conjugated polymers as they are usually insoluble or infusible because of the rigidity of the main polymer chain and strong intermolecular forces between polymer chains. One way to improve the processibility of these polymers is to prepare a soluble precursor which can then be converted into a rigid conjugated polymer, as can be done with poly(p-penylenevinylene) (PPV) (A) [A green yellow emitter, prepared by the sulfonium precursor route: P. L Burn, D. D. C. Bradley, R. H. Friend, D. A. Halliday, A. B. Holmes, R. W. Jackson and A. Kraft, J. Chem Soc., Perkin Trans., 1992, 1, 3225]. Another way is to generate a fully conjugated material while increasing solubility by attaching bulky and flexible alkyl or alkoxy groups onto the main chain thereby weakening the intermolecular forces (as shown in the case of alkylxe2x80x94or alkoxy-substituted PPV in (B) and (C)). A third way is to attach or insert a photoluminescent chromophore to a flexible polymer chain since the flexible chain segments will enhance the solubility in conventional organic solvents. This has been shown in the case of a block copolymer consisting of xcfx80-conjugated active blocks sandwiched between non-active flexible blocks [R. Gill, G. Hadziioannou, J. Herrema, G. Malliaris, R. Wieringa, J. Wildeman, WPI Acc. . No.94-234969; Z. Yang, I. Sokolik, F. E. Karasz, Macromolecules, 1993, 26(5), 1188Sumitomo Chem. Co. Ltd., JP 5320635]. 
In order to improve the performance of polymer LEDs, the luminescent polymer needs to be used in association with a charge transport polymer. Conventionally, charge transport materials may be used as single layers between the emitting layer and the electrodes. Alternatively, blends may be used.
Thus, prior art polymers used in optical devices suffer from susceptibility to solvents and morphological changes owing to low glass transition temperatures. Moreover, when molecular electron transport materials are used in such optical devices, problems involving the aggregation and recrystallisation of the material may lead to device breakdown.
In one aspect, the present invention provides a semiconductive polymer capable of luminescence in an optical device, such as a photoluminescent or electroluminescent device. The polymer comprises a luminescent film-forming solvent processible polymer which contains cross-linking so as to increase its molar mass and to resist solvent dissolution, the cross-linking being such that the polymer retains semiconductive and luminescent properties.
By increasing the molar mass of the polymer the deleterious effects of susceptibility to solvents and morphological change owing to low glass transition temperatures are avoided. Surprisingly, the cross-liked polymers retain their semiconductive and luminescence properties. Luminescent and electroactive polymer thin films such as those used in optical devices may therefore be stabilised. Because the thin films resist dissolution in common solvents this enables deposition of further layers of, for example, electroactive polymer films by solvent coating techniques thereby facilitating device manufacture. The cross-linked semiconductive polymers retain all their desirable luminescence properties and have the advantage of exhibiting enhanced morphological stability under device operation.
The cross-linking may be formed in the semiconductive polymer by thermal cross-linking, chemical cross-linking or photochemical cross-linking. Cross-linking methodology for polymers is well-known. For example, the cross-linking of polymers for photoresists by thermal, chemical and photochemical methods have been reviewed; (S. Paul, Cross Linking Chemistry of Surface Coatings, in Comprehensive Polymer Science, G. Allen (Ed.), Pergamon, Oxford, 1989, Vol. 6, Ch. 6, pp. 149-192; S. R. Turner and R. C. Daly, Photochemical and Radiation-sensitive Resists, in Comprehensive Polymer Science, G. Allen (Ed.), Pergamon, Oxford, 1989, Vol. 6, Ch. 7, pp. 193-225; S. P. Pappas, Photocrosslinking in Comprehensive Polymer Science, G. Allen (Ed.), Pergamon, Oxford, 1989, Vol. 6, Ch. 5, pp. 135-148). In addition, an example of cross linking of polymers through ring opening metathesis polymerization of cyclooctene-5-methacrylate was reported by B. R. Maughon and R. H. Grubbs, (Polym. Prepr., 1995, 36, 471-472).
A particularly useful example of thermal cross-linking involves the use of azide groups usually attached to the polymer main chain by a spacer group. At a temperature typically in the range of 80xc2x0 C. to 250xc2x0 C. the aliphatic azide will either form a pyrazoline adduct with a double bond or decompose to form a highly reactive nitrene which can then form cross-links with other polymers. An aryl azide will behave similarly in the range 20xc2x0 C. to 250xc2x0 C. The spacer is advantageously non-rigid. Preferably the spacer comprises xe2x80x94(CH2)nxe2x80x94 or xe2x80x94(CH2)nxe2x80x94Arxe2x80x94 in which n is an integer preferably in the range 2 to 20 and Ar is an aryl group, preferably a phenylene group. A good example of such a spacer is a xe2x80x94(CH2)11xe2x80x94 group.
Chemical cross-linking may be effected using diisocyanates or activated dicarboxylic acid derivatives to react with terminal functional groups (e.g. xe2x80x94OH) on the soluble polymer. In this way urethane or ester linkages can be created. Alternatively, a low molecular weight bifunctional or polyfunctional compound (e.g. an epoxy resin) can be blended with the solvent processible polymer for the purpose of reacting chemically with existing functional groups (e.g. amino etc) in the polymer main chain or on the side chains of the polymer. Suitable cross-linking agent include formaldehyde or other aldehydes, bis or polyfunctional azides such as 1,6-bisazidohexane, and polyisocyanates.
Photochemical cross-linking may be effected by any side chain substituent capable of becoming activated upon irradiation with light of appropriate energy, usually UV light. For example, cinnamate esters will undergo [2+2]-cycloaddition under appropriate conditions, typically irradiation of the polymer film at ambient temperature with a medium pressure Hg lamp. Also, photolysis of alkyl or aryl azides over a wide temperature range, preferably xe2x88x9250xc2x0 C. to +50xc2x0 C., can generate reactive nitrene intermediates which can cross-link the polymer.
The luminescent film-forming polymer and the cross-linked form thereof according to the present invention may be luminescent either by virtue of a luminescent main chain or a luminescent side chain. The polymer may comprise any such film-forming polymer, including copolymers and oligomers. The luminescent main chain polymers have been described in PCT GB 90/00584 and PCT GB91/01420, for example. Such polymers include poly (arylene vinylene) derivatives. Particularly useful poly (arylenevinylene) polymers in the present invention include polymers of general formula B and which carry cross-linkable functionality as an attachment. electroluminescent polyarylenes are also particularly useful in the present invention, including polyheteoarylenes, especially the polythiophenes. Polythiophene copolymers are known to be capable of luminescence and substituted poly(3-alkyl thienylenes) are preferred.
Statistical copolymers of substituted poly(3-alkylthienylene) s containing regioregular head to tail linkages can be made according to K. A. Murray, S. C. Moratti, D. R. Baigent, N. C. Greenham, K. Pichler, A. B. Holmes and R. H. Friend, in Snyth. Met., 1995, 69, 395-396 and then cross-linked. The side chain alkyl substituents or a fraction thereof carry functionality which can be employed in chemical, photochemical or thermal cross-linking processes.
Further examples of polymers having a luminescent main chain are those which have the electroluminescent segments in scheme 2 below forming part of the polymer main chain. In a preferred embodiment of the invention, the polythiophene copolymer is of the general formula 
in which Rxe2x80x2 is a solubilising group, Rxe2x80x3 is a spacer group cross-linking the main chain to another polymer, and x, y and n are each integers, wherein x:y is in the range 19:1 to 1:2 and n is in the range 3 to 100.
Preferably, Rxe2x80x2 is xe2x80x94C6H13.
Where the polymer includes a luminescent side chain, this side chain may incorporate any luminophoric group such as a species containing at least 3 unsaturated units in conjugation. Preferably the luminescent side chain comprises a distyryl benzene. Where the polymer includes a luminescent side chain, there is no need for the main chain of the polymer itself to be luminescent but the polymer should be transparent to the emitted light. Various polymers may therefore be used to form the main chain. Especially useful polymers include polystyrenes, polyacrylates, polysiloxanes, and polymethacrylates which are preferred. Polymethacrylates are discussed in further detail below.
In one embodiment of the invention, the polymer further comprises a charge transport segment which is present in the polymer main chain or covalently linked thereto in a charge transport side chain.
In a further aspect of the invention a polymer is provided which is capable of charge transport, preferably electron transport, in an optical device such as an electroluminescent device. The polymer comprises a film-forming polymer which is solvent processible or formed from a processible precursor polymer and which includes a charge transport segment in the polymer main chain or covalently linked thereto in a charge transport side chain.
The polymers may be used as both charge transporting and/or electroluminescent materials in polymer light emitting devices. The polymers may therefore include charge transport functional segments and electroluminescent functional segments either as a side chain group or in the main chain of the polymer. Precursor polymers leading, after a conversion step, to intractable final polymers may be used, as well as fully processible polymers. Each type of polymer can have specific advantages in processing multi-layered structures.
The charge transport segment may comprise the moiety Ar1xe2x80x94Hwetxe2x80x94Ar2in which Ar1 and Ar2 are the same or different from one another and are aromatic units. Examples of these aromatic units are set out below in Scheme 1. Het is a heteroaromatic ring, the electronic structure of which favours charge transport. Examples include oxidiazole, thiadiazole, pyridine, pyrimidine and their benzo-fused analogues such as quinoline. Heteroaromatic rings which are electron deficient and therefore enhance charge injection and transport are generally useful. 
Ar1, Ar2 are aromatic, heteroaromatic, fused aromatic derivatives thereof, or double bonds: 
R1-19 and R1-12 are groups selected from the groups consisting of hydrogen and halogen atoms and cyano, alkyl and alkoky side chains.
Scheme 1 Representative charge transport segments 
Scheme 2 Representative Electroluminescent Segments
The electroluminescent segments may comprise conjugated photoluminescent chromophore segments as illustrated in Scheme 2.
The side chain (co)polymer consists of any backbone polymer containing side chain modifications with luminescent and/or electron transporting segments. A typical example is a poly(methacrylate) that contains charge transport segments and/or luminescent segments in the pendant side group as shown in Scheme 3.
The side chain polymer may contain an optional third functional segment that will play a cross-linking role so as to improve the stability of the poly(methacrylate) i.e. by raising the glass transition temperature (tg). The third segment may be a chemically cross-linkable group such as an epoxide, a thermally cross-linkable group such as an azide, or a photocross-linkable group such as a cinnamate or a stilbene group. 
Scheme 3 Illustration of a side chain copolymer (using poly(methacrylate) as an example)
The main chain polymers and copolymers referred to herein are (co) polymers that contain transport segments and/or electroluminescent segments along the polymer or copolymer backbone with or without flexible spacers as illustrated by a representative example in Scheme 4.
The polymers described in the present invention are particularly suitable for use as electron transport layers in a multilayer LED device either as a blend with another electroluminescent polymer or as one of the components in a copolymer with another electroluminescent segment. This improves both the internal quantum efficiency and device performance. 
Scheme 4 Representative Main Chain Polymer Structures 
Scheme 5: Synthesis of oxadiazole monomers
Poly (methacrylates) have many advantages such as high transparency, high resistance to chemicals, and good mechanical strength. It is also relatively easy to synthesise high molecular weight polymers as well as multi-functional copolymers.
To illustrate this general concept, a range of aromatic oxadiazole bonded polymers [especially poly(methacrylates)] have been synthesised and investigated incorporating monomers as shown in Scheme 5. These (co)polymers can be used in association with emissive polymers in different ways (single layer, blended layer and copolymer layer) to give devices with improved performance. 
Scheme 6 A Poly(methacrylate) Containing a Blue Light Emitting Side Chain
In a previous patent application (PCT/GB93/02856) a range of poly(methacrylate) derivatives containing chromophores D featuring blue emission were synthesised. The chromophoric groups F,G,H,I comprised two or three conjugated aryl rings (distyrylbenzene units) attached to the poly(methacrylate) chain via covalent linkages. This is a representative example of the numerous possibilities for blue side chain modified light emitting polymers. Crosslinking and copolymerisation with polymers carrying charge transporting segments make these materials particularly attractive candidates for blue light emission.
The polymer capable of charge transport is generally used in an optical device as a functional polymer layer between an electroluminescent polymer layer and a charge injection electrode. This layer plays a role in enhancing charge and especially electronic injection from the metal electrode (usually a cathode) and charge transport. The polymer may balance the charge injection in a multi-layer polymer LED with improvement of device performance.
In a further aspect, the present invention provides use of a polymer as described above in an optical device, preferably an electroluminescent device. The present invention also provides an optical device which comprises a substrate and a polymer as defined above supported on the substrate. The optical device is preferably an electroluminescent device. Typically, the polymer is used in such devices as a thin film. In operation the cross-linked semiconductive polymers retain desirable luminescent properties and have the advantage of exhibiting enhanced morphological stability.